1. Field of the Invention
This invention relates to an apparatus for scanning a sheet-shaped recording medium by a light beam and recording an image thereon or reading out an image therefrom. This invention particularly relates to an apparatus for scanning a sheet-shaped recording medium, which is provided with an improved sub-scanning conveyance system wherein sub-scanning is conducted by moving the recording medium in one direction at a predetermined speed during two-dimensional scanning of the recording medium by a light beam.
2. Description of the Prior Art
An image scanning method for recording an image in a recording medium or for reading out an image therefrom by two-dimensionally scanning the recording medium by a light beam is widely used in facsimile devices and the like. In the method, two-dimensional scanning is conducted by deflecting a light beam by a light deflector in one direction (main scanning direction) to form a scanning line on the recording medium and by mechanically moving the recording medium in a direction (sub-scanning direction) normal to the main scanning direction, thereby forming many scanning lines in parallel with each other.
As disclosed, for example, in U.S. Pat. No. 4,258,264 and Japanese Unexamined Patent Publication No. 56(1981)-11395, the aforesaid image scanning method is also employed for reading out a radiation image stored in a stimulable phosphor sheet or for reproducing the read-out radiation image stored in a recording medium in a radiation image recording and reproducing system. In the radiation image recording and reproducing system, it is necessary to regularly conduct scanning in the main scanning direction at a predetermined period and to carry out movement of the recording medium in the sub-scanning direction at a predetermined speed. That is, when the recording medium is moved at a low speed in the sub-scanning direction and, at the same time, a light beam is deflected in the main scanning direction normal to the sub-scanning direction on the recording medium, the recording medium must be precisely moved at the predetermined speed. Otherwise, recording nonuniformity or read-out nonuniformity arises, and it becomes impossible to obtain a reproduced image accurately representing the image density. Therefore, high accuracy is required for scanning in the sub-scanning direction, necessitating high-precision expensive holding means and movement mechanism for the recording medium.
In order to move the recording medium as described above, various mechanisms have heretofore been used. In one arrangement, for example, the sheet-shaped recording medium is closely contacted with the surface of a drum by using clamping devices or pinch rolls, and the drum is rotated. In another arrangement, a suction box is positioned at a part inside of a drum, and the drum is rotated with the recording medium closely contacted with a part of the drum by suction. In still another mechanism, a suction box is positioned inside of a perforated belt, and the belt is moved with the recording medium placed thereon. In these mechanisms, since the clamping devices or the suction box is required for closely contacting the recording medium with the drum or the belt, the mechanism becomes complicated and the apparatus becomes large and expensive.
Further, in the case where a suction box is used, there arise various problems in practical use such as the noise and vibration of a blower for suction, the fluctuation of the suction pressure, and the like. Particularly in a scanning apparatus for image recording or read-out wherein the recording medium must be moved precisely and a precise scanning optical system is incorporated, vibration and fluctuation of the suction pressure cause the movement speed of the recording medium to fluctuate and adversely affect the recording or read-out operation.
In an image read-out scanning apparatus for scanning a recording medium carrying an image stored therein by a light beam and for detecting light carrying the image obtained thereby, a light guide member is generally positioned near the recording medium. In this case, in order to improve the light guiding efficiency, it is generally desired that the light guide member be positioned as close to the recording medium as possible. However, when the conventional clamping devices or pinch rolls are used, it is not always possible to position the light guide member close to the recording medium because of the presence of mechanisms protruding in the vicinity of the clamping devices or the pinch rolls.